Negative electrode active material for nonaqueous electrolyte battery, method of producing the same, nonaqueous electrolyte battery and battery pack

ABSTRACT

According to one embodiment, a negative electrode active material for nonaqueous electrolyte battery includes a titanium oxide compound having a crystal structure of monoclinic titanium dioxide. When a monoclinic titanium dioxide is used as the active material, the effective capacity is significantly lower than the theoretical capacity though the theoretical capacity was about 330 mAh/g. The invention comprises a titanium oxide compound which has a crystal structure of monoclinic titanium dioxide and a (001) plane spacing of 6.22 Å or more in the powder X-ray diffraction method using a Cu—Kα radiation source, thereby making an attempt to improve effective capacity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2009-073123, filed Mar. 25, 2009,the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a negative electrodeactive material for nonaqueous electrolyte battery, a method ofproducing the negative electrode active material, a nonaqueouselectrolyte battery and a battery pack.

BACKGROUND

Since the nonaqueous electrolyte battery such as lithium ion secondarybattery have high energy densities, they are expected to be used forpower sources for hybrid vehicles, electric cars, and an uninterruptiblepower supply for base stations for portable telephone, and the like.Therefore, the nonaqueous electrolyte battery is desired to have otherperformances such as rapid charge/discharge performances and long-termreliability. For example, a nonaqueous electrolyte battery enablingrapid charge/discharge not only remarkably shortens the charging timebut also makes it possible to improve performances of the motive forceof a hybrid vehicle and the like and to efficiently recover theregenerative energy of them.

In order to enable rapid charge/discharge, it is necessary thatelectrons and lithium ions can migrate rapidly between the positiveelectrode and the negative electrode. When a battery using a carbonbased material in the negative electrode repeats rapid charge/discharge,this causes dendrite precipitation of metal lithium on the electrode,raising the fear as to heat generation and fires caused by internalshort circuits.

In light of this, a battery using a metal composite oxide in place of acarbonaceous material in the negative electrode has been developed.Particularly, a battery using a titanium oxide as the negative electrodeactive material enables stable rapid charge/discharge and also has alonger life than those using a carbonaceous material.

However, titanium oxide has a higher potential than carbonaceousmaterial relative to metal lithium. Further, titanium oxide has a lowercapacity per mass. Thus a battery using titanium oxide as the negativeelectrode active material has a problem that the energy density islower.

For example, the potential of the electrode using titanium oxide isabout 1.5 V based on metal lithium and is nobler than that of theelectrode using carbonaceous material. The potential of titanium oxideis due to the redox reaction between Ti³⁺ and Ti⁴⁺ when lithium iselectrochemically inserted and released and is therefore limitedelectrochemically. Further, there is the fact that the inserted andreleased of lithium ions by rapid charge/discharge is possible at anelectrode potential as high as about 1.5 V. It is thereforesubstantially difficult to drop the potential of the electrode toimprove energy density.

As to the capacity of the battery per unit mass, the theoreticalcapacity of titanium dioxide having an anatase structure is about 165mAh/g and the theoretical capacity of a lithium-titanium composite oxidesuch as Li₄Ti₅O₁₂ is also about 170 mAh/g. On the other hand, thetheoretical capacity of a general graphite type electrode material is385 mAh/g or more. Therefore, the capacity density of titanium oxide issignificantly lower than that of the carbon type material. This is dueto a reduction in substantial capacity because there are only a smallnumber of equivalent lithium-absorbing sites in the crystal structureand lithium tends to be stabilized in the structure.

In light of this, monoclinic titanium dioxide has recently attractedattention (see R. Marchand, L. Brohan, M. Tournoux, Material ResearchBulletin 15, 1129 (1980)). Lithium titanate having a spinel structuresuch as Li₄Ti₅O₁₂ can release/insert 3 lithium ions per unit chemicalformula. Therefore, the number of lithium ions per titanium ion is ⅗ anda theoretical maximum of 0.6. On the other hand, in monoclinic titaniumdioxide, the number of lithium ions per titanium ion which can bereleased/inserted is a maximum of 1.0. Accordingly, the theoreticalcapacity of monoclinic titanium dioxide is about 330 mAh/g. Therefore,it is expected that monoclinic titanium dioxide may be used as ahigh-capacity negative electrode active material.

For example, JP-A 2008-34368 (KOKAI) discloses a lithium ion storagebattery using titanium oxide TiO₂ having a bronze structure as thenegative electrode active material. JP-A 2008-34368 (KOKAI) disclosesthat the substantial capacity of a lithium ion storage battery using thetitanium oxide TiO₂ as the electrode active material and a lithium metalas the counter electrode is about 200 mAh/g (for example, Paragraph 0029and FIG. 4).

JP-A 2008-117625 (KOKAI) discloses a lithium secondary battery using, asthe active material, titanium dioxide having crystal structure of abronze type titanic acid. JP-A 2008-117625 (KOKAI) discloses that alithium secondary battery (coin type cell) using the titanium dioxide asthe active material and a lithium metal as the counter electrode has aninitial insertion and release capacity of 160 to 170 mAh/g based on themass of the active material (for example, Paragraphs 0053 and 0057).

Although, the theoretical capacity in the case of using monoclinictitanium dioxide as the active material is about 330 mAh/g, thesubstantial capacities disclosed in JP-A 2008-34368 (KOKAI) and JP-A2008-117625 (KOKAI) are significantly lower than the theoreticalcapacity. Therefore, if the titanium dioxide described in JP-A2008-34368 (KOKAI) or JP-A 2008-117625 (KOKAI) are used as the activematerial, it is difficult to raise the capacity further as compared, forexample, with the case of using lithium titanate having a spinelstructure which has the theoretical capacity of 170 mAh/g.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a typical view of a crystal structure of TiO₂(B) in a firstembodiment;

FIG. 2 is a sectional view of a flat type nonaqueous electrolyte batteryaccording to a third embodiment;

FIG. 3 is an enlarged sectional view of a part A in FIG. 2;

FIG. 4 is a partly broken perspective view of another flat typenonaqueous electrolyte battery according to a third embodiment;

FIG. 5 is an enlarged sectional view of a part B in FIG. 4;

FIG. 6 is an exploded perspective view of a battery pack according to afourth embodiment;

FIG. 7 is a block diagram showing an electric circuit of a battery packshown in FIG. 6;

FIG. 8 is a powder XRD pattern of titanium dioxide of Example 1;

FIG. 9 is a powder XRD pattern of titanium dioxide of Example 2;

FIG. 10 is a diagram showing charge/discharge curves in Examples 1 and 2and Comparative Example 1; and

FIG. 11 is a graph showing the relation between the d₀₀₁ of TiO₂(B) andfirst cycle discharge capacity.

DETAILED DESCRIPTION

In general, according to one embodiment, a negative electrode activematerial for nonaqueous electrolyte battery is provided. The activematerial includes a titanium oxide compound having a crystal structureof a monoclinic titanium dioxide and satisfying the following equation(I):d₀₀₁≧6.22 Å  (I):

wherein d₀₀₁ is the spacing of the (001) plane which is measured by thepowder X-ray diffraction method using a Cu-Kα radiation source.

In general, according to another embodiment, a method of producing anegative electrode active material for nonaqueous electrolyte battery isprovided. The method includes: reacting an acid with at least onecompound selected from the group consisting of potassium titanate,sodium titanate and cesium titanate to exchange alkali cations thereoffor protons, thereby obtaining a proton-exchanged compound; andheat-treating the proton-exchanged compound to synthesize a titaniumoxide compound which has a crystal structure of a monoclinic titaniumdioxide. The titanium oxide compound produced by the method is satisfiedthe following equation (I) and (II):d₀₀₁≧6.22 Å  (I)I(200)/I(001)≦0.5  (II)

wherein d₀₀₁ is the spacing of the (001) plane; I(001) is the peakintensity of the (001) plane; and I(200) is the peak intensity of the(200) plane; which are measured by the powder X-ray diffraction methodusing a Cu-Kα radiation source.

In general, according to another embodiment, a nonaqueous electrolytebattery is provided. The battery includes a positive electrode; anegative electrode comprising the negative electrode active materialaccording to above embodiment; and a nonaqueous electrolyte.

In general, according to another embodiment, a battery pack comprisingthe nonaqueous electrolyte battery is provided.

(First Embodiment)

A negative electrode active material for nonaqueous electrolyte batteryaccording to a first embodiment comprises a titanium oxide compoundhaving a crystal structure of a monoclinic titanium dioxide andsatisfying the following equation (I):d₀₀₁∝6.22 Å  (I)

wherein d₀₀₁ is the spacing of the (001) plane which is measured by thepowder X-ray diffraction method using a Cu-Kα radiation source.

The problem that monoclinic titanium dioxide has an effective capacityless than the theoretical capacity is considered to be caused by smallereffective mobile Li ions because the diffusibility of Li ions in a solidis low though there are many sites having the possibility of becoming Lihosts in the crystal structure. The symmetric properties such as a spacegroup in monoclinic titanium dioxide may be varied because strains aregenerated depending on the amount and type of intercalation. Theinventors first found that the diffusibility of Li ions in a solid isimproved to thereby improve the effective capacity while maintaining anelectrode potential close to 1.5 V based on lithium by using a negativeelectrode active material comprising a titanium oxide compound accordingto the embodiment. As a result, a nonaqueous electrolyte battery can beattained which has a high energy density and excellent repeatcharge/discharge performances.

Here, the crystal structure of monoclinic titanium dioxide is designatedas TiO₂(B). The crystal structure represented by TiO₂(B) primarilybelongs to the space group C2/m and has a tunnel structure as shown inFIG. 1. The details of the crystal structure of TiO₂(B) are described inR. Marchand, L. Brohan, M. Tournoux, Material Research Bulletin 15, 1129(1980).

The crystal structure represented by TiO₂(B) has a structure in which,as shown in FIG. 1, a titanium ion 3 and an oxide ion 2 constitute askeleton structure part 1 a and skeleton structure parts 1 a arealternately arranged. A void 1 b is formed between the skeletonstructure parts 1 a. This void 1 b may be a host site for theintercalation (or insertion) of different atom species. In TiO₂(B), itis said that host sites which can absorb and release different atomspecies exist on the surface of the crystal. Lithium ions are insertedinto and released from these host sites, whereby TiO₂(B) can reversiblyabsorb and release lithium ions.

When lithium ions are inserted into the voids 1 b, Ti⁴⁺ constituting theskeleton is reduced to Ti³⁺ to thereby keep the electrical neutrality ofthe crystal. Because TiO₂(B) has one Ti⁴⁺ per chemical formula, amaximum of one lithium ion can be theoretically inserted per chemicalformula. Therefore, a titanium oxide compound having a TiO₂(B) crystalstructure may be represented by the formula Li_(x)TiO₂ (0≦x≦1). In thiscase, this titanium oxide compound provides a theoretical capacity of335 mAh/g, which is about two times that of titanium dioxide describedin JP-A 2008-34368 (KOKAI) and JP-A 2008-117625 (KOKAI).

The inventors have found that the mobility of Li ions in a solid isimproved by increasing the size of the above void 1 b in the titaniumoxide compound having a TiO₂(B) crystal structure and as a result, theeffective capacity of the electrode and repeat charge/dischargeperformances can be improved.

The size of the void 1 b can be adjusted by controlling d₀₀₁ of thecrystal lattice. d₀₀₁ can be calculated from a (001) plane peak whichappears in the vicinity of 2θ=14.5° in the pattern obtained by thepowder X-ray diffraction method using a Cu-Kα radiation source. Here,the term “in the vicinity of 2θ=14°” is intended to mean a range of2θ=14°±2°, namely 2θ is from 12° to 16°.

The titanium oxide compound according to this embodiment has d₀₀₁ of6.22 Å or more. When d₀₀₁ is 6.22 Å or more, a high effective capacityand excellent repeat charge/discharge performances can be provided. Whend₀₀₁ is less than 6.22 Å, the mobility of Li ions inserted into thelattice crystal is significantly reduced, leading to reduction incharge/discharge efficiency and effective capacity.

Further, d₀₀₁ is preferably 6.50 Å or less. When d₀₀₁ is 6.50 Å or less,the crystal structure is stable and good repeat charge/dischargeperformances are obtained.

Moreover, the titanium oxide compound according to this embodimentpreferably has a peak intensity ratio defined by the following equation(II).I(200)/I(001)≦0.5  (II)where I(001) is the peak intensity of the (001) plane and I(200) is thepeak intensity of the (200) plane which appears in the vicinity of2θ=15° in the pattern obtained by the powder X-ray diffraction methodusing a Cu-Kα radiation source. The peak of the (200) plane lie adjacentto the peak of the (001) plane in the X-ray diffraction pattern. Here,the term “in the vicinity of 2θ=15°” is intended to mean a range of2θ=15°±2°, namely 2θ is from 13° to 17°.

The peak intensity ratio I(200)/I(001) is preferably 0.5 or less. Whenthe peak intensity ratio I(200)/I(001) is 0.5 or less, a proper amountof water is kept in the crystal structure, so that the volume of lithiumion host sites is sufficiently secured. At this time, the projected areaof the void 1 b can reach the maximum value. This enables more smoothmigration of lithium ions, leading to more improvements in effectivecapacity and repeat charge/discharge performances. Further, because thegeneration of different crystal phases is limited, an improvement incharge/discharge efficiency can be expected.

Here, the method of powder X-ray diffraction measurement will beexplained.

First, an target sample is ground until the average particle diameterreaches about 5 μm. The ground sample is filled in a holder part whichis formed on a glass sample plate and has a depth of 0.2 mm. At thistime, much care is necessary to fill the holder part fully with thesample. Further, special care should be taken to avoid cracking andformation of voids caused by insufficient filling of the sample.

Then, a separate glass plate is used to smooth the surface of the sampleby sufficiently pressing the separate glass plate against the sample.Much care should be taken to avoid too mach or too little amount of thesample to be filled, thereby preventing any rises and dents in the basicplane of the glass holder. The glass plate filled with the sample ismounted on the powder X-ray diffractometer.

Then, the measurement is carried out by using Cu-Kα rays to determinethe position and the intensity of the peak of the (001) plane whichappears in the vicinity of 2θ=14°. d₀₀₁ can be calculated from theobtained position of the peak. Generally, the plane spacing can becalculated by the following Bragg reflection condition equation:2dsinθ=nλwherein d: plane spacing, λ: wavelength of X-ray to be used, n: integerand θ=angle of a peak position.

When the sample has a high orientation, there is the possibility of ashift of a peak position and variation in intensity ratio, depending onhow the sample is filled. Such a sample is made into a pellet form formeasurement. The pellet may be a compressed powder body 10 mm indiameter and 2 mm in thickness, which is manufactured by applying apressure of about 250 MPa to the sample for 15 minutes. The obtainedpellet is set to an X-ray diffractometer to measure the surface. Themeasurement using such a method eliminates a difference in the resultsof the measurement between operators, enabling high reproducibility.

Further, the intensity of the peak of the (200) plane which appears inthe vicinity of 2θ=15° in the pattern obtained by the powder X-raydiffraction is determined. The intensity of the peak of the (200) planeis used to calculate the intensity ratio I(200)/I(001). When these twopeaks (i.e. diffraction curves) are separated, the ratio is calculatedfrom intensity of each peak.

In the case where the peak of the (001) plane and the peak of the (200)plane cannot be easily separated from each other, the obtained data maybe treated by a computer using specified software to obtain each peakintensity. When these two peaks cannot be separated even by suchtechniques, the intensity ratio I(200)/I(001) is regarded as 0 in thisembodiment.

There is the case where the peak intensities of the (001) plane and(200) plane cannot be detected. This is considered to be due to theinfluence of, for example, orientation during the growth of crystals.When the peak of the (001) plane is not detected, the plane spacing ofthe (001) plane is for example, calculated using the index of otherplane which may be used for estimation the plane spacing of the (001)plane by geometrical computation. In the simplest example, the planespacing of the (002) plane is used for estimation. In this case,d(001)=d(002)×2. In the case where the peak of the (200) plane is notdetected, it may also be likewise calculated from the index of otherplane.

<Particle Diameter and BET Specific Surface Area>

The active material of the electrode is said to enable more rapidcharge/discharge and has higher capacity with increase in the contactsurface with the electrolytic solution and also with increase in thenumber of host sites. For this, there is an attempt to improve the rapidcharge/discharge performance by grinding titanium oxide.

The average particle diameter of the titanium oxide compound in thisembodiment may be varied corresponding to desired battery performanceswithout any particular limitation. The BET specific surface area of thetitanium oxide compound in this embodiment is preferably 6 to 200 m²/gthough no particular limitation is imposed.

If the specific surface area is 6 m²/g or more, the contact area withthe electrolytic solution can be secured. If the specific surface areais 200 m²/g or less, on the other hand, the reactivity with theelectrolytic solution is not too high and therefore, the lifeperformances can be improved. Further, this limited specific surfacearea allows a slurry containing the active material to be coated withfacility in the production of an electrode, which will be describedlater.

Here, in the measurement of the specific surface area, a methodcomprising the following step is used:

a molecule of which the adsorption occupying area is known is made toadsorb to the surface of the powder particle at the temperature ofliquid nitrogen; and

the specific surface area of the powder particle is calculated from theamount of the molecule adsorbed on it. A method that is most frequentlyused to obtain the specific surface area is the BET method. The BETmethod uses the low-temperature and low-humidity physical adsorption ofan inert gas and is based on the well-known theory for the calculatingspecific surface area. The theory is obtained by extending the Langmuirtheory which is monolayer adsorption theory to multilayer adsorption.The specific surface area calculated in this manner is referred to asthe “BET specific surface area”.

(Second Embodiment)

A method of producing a negative electrode active material fornonaqueous electrolyte battery according to the above first embodimentwill be explained in detail.

A titanium oxide compound according to the first embodiment is obtainedby sintering of a proton-exchanged compound, which is a startingmaterial. The inventors have succeeded in obtaining a titanium oxidecompound which has a TiO₂(B) structure and a plane spacing d₀₀₁ of 6.22Å or more by properly adjusting the sintering condition.

As the starting material, alkali titanate compounds such as Na₂Ti₃O₇,K₂Ti₄O₉ and Cs₂Ti₅O₁₂ may be used, though the embodiment is not limitedby these compounds.

The alkali titanate compound which may be the starting material isprepared by a usual solid phase reaction method. For example, the alkalititanate compound may be synthesized by blending a raw material such asoxide, carbonate and the like in a proper stoichiometric ratio and byheating the mixture. Alternatively, a reagent of a commerciallyavailable alkali titanate compound may be used.

First, a powder of an alkali titanate compound is washed with distilledwater to remove impurities. Then, 0.5 to 2 M of an acid such ashydrochloric acid, nitric acid or sulfuric acid is added to the alkalititanate compound powder, followed by stirring. Alkali cations of thealkali titanate compound are exchanged for protons by this acidtreatment to thereby obtain a proton-exchanged compound. The acidtreatment is preferably carried out until protons are sufficientlyexchanged.

The time required for the acid treatment is preferably 24 hours or moreand more preferably 1 to 2 weeks when the acid treatment is carried outat ambient temperature (about 25° C.) by using about 1 M of hydrochloricacid. The acid solution is preferably exchanged for a new one, forexample, every 24 hours to ensure the progress of proton exchange.

After the proton exchange is finished, an alkaline solution such as anaqueous lithium hydroxide solution may be added to neutralize residualacids. After the proton exchange is completed, the reaction product iswashed with distilled water. Although no particular limitation isimposed on the degree of washing, the reaction product is washed untilthe pH of washed water falls in a range from 6 to 8. Then, the productis dried to obtain an intermediate product, a proton-exchanged compound.Here, the process of neutralization washing of residual acid and thedrying process may be omitted and the obtained proton-exchanged compoundmay be subjected to a heat treating step.

Such an ion exchange method enables alkali cations to be exchanged forprotons without destroying the crystal structure of the alkali titanatecompound.

Preferably, the raw material compound is ground by a ball mill beforethe ion exchange method so that proton exchange is smoothlyaccomplished. As to grinding conditions for a container having an areaof 100 cm², zirconia balls having a diameter of about 10 to 15 mm areused and the ball mill is rotated at 600 to 1000 rpm for about 1 to 3hours. When the rotating time is less than one hour, this is undesirablebecause the raw material is ground only insufficiently. Further, if theraw material is ground for a time as long as 3 hours or more, it isphase-separated into a compound different from a target product becausea mechanochemical reaction proceeds, which is undesirable.

Next, the obtained proton-exchanged compound is heat-treated to obtain atitanium oxide compound as a target product. The heating condition isproperly selected such that d₀₀₁ of the TiO₂(B) crystal is 6.22 Å ormore. The inventors have found that optimum heating conditions differdepending on the composition of the starting material, particlediameter, crystal form and proton exchange condition. It is thereforenecessary to properly determine the optimum heating condition inaccordance with the starting material. Even if any starting material isused, a titanium oxide compound having a plane spacing of (001) planed₀₀₁ of 6.22 Å or more can be obtained by controlling, for example, theheating temperature and time.

In the method according to this embodiment, an electric furnace isheated in advance to perform a precise heat treatment. The sample isintroduced into the electric furnace after the electric furnace reachesa set temperature. After the sample is heated under heating conditionswhich is determined specifically for the sample, it is taken out of thefurnace rapidly, followed by rapid cooling in air. This enables theheating condition to be set exactly and therefore, restrains theproduced TiO₂(B) crystal from being excessively dehydrated.

The heating temperature is preferably in a range from 300° C. to 500°C., more preferably in a range from 350° C. to 400° C., because thecrystal plane spacing is easily controlled. When the heating temperatureis 300° C. or more, this is preferable because a dehydration reactionrapidly proceeds, which improves crystallinity, bringing aboutimprovements in electrode capacity, charge/discharge efficiency andrepeat performances of the battery. When the heating temperature is 500°C. or less, this is preferable because the progress of a dehydrationreaction is not too fast and therefore, the (001) plane of the TiO₂(B)crystal structure is not decreased too much. Further, when the heatingtemperature is 500° C. or less, this restrains the generation oftitanium dioxide having anatase structure which is an impurity phase anddeteriorates the electrode performance.

The appropriate heating temperature and heating time can be determinedby the following procedures. The proton-exchanged compound is heated ateach of five different temperatures set at intervals of 25° C. in atemperature range from 300° C. to 400° C. for 3 hours. With regard tothe obtained titanium oxide compound, the peak intensity of the (001)plane which appears in the vicinity of 2θ=14° is measured. Then, usingthe peak intensity, calculate d₀₀₁ value. The optimum heatingtemperature is determined from this d₀₀₁ value.

The titanium oxide compound which has been heat-treated under anappropriate heating condition according to the embodiment has a (001)plane spacing d₀₀₁ of 6.22 Å or more. Such a titanium oxide makes itpossible to attain a charge/discharge capacity of 220 mAh/g, which ishigher by 10 to 30% than that of titanium dioxide, which has a TiO₂(B)structure and is produced by a conventionally known method.

Before this application, it was not known that heating conditions affectlattice plane spacing, as well as the lattice plane spacing affects theelectrode performance. In conventional methods of synthesizing titaniumdioxide having a TiO₂(B) structure, it is considered that the heatingconditions for a proton-exchanged compound are determined tosufficiently improve the crystallinity of TiO₂(B). However, if thecrystallinity of TiO₂(B) becomes too high, the dehydration reaction ofthe crystal lattice is caused excessively, resulting in a narrowerlattice plane spacing. Particularly, the plane spacing d₀₀₁ calculatedfrom a diffraction curve corresponding to the (001) plane is less than6.22 Å (see R. Marchand, L. Brohan, M. Tournoux, Material ResearchBulletin 15, 1129 (1980)).

For example, the heating conditions described in JP-A 2008-34368 (KOKAI)are 400° C. and 3 hours. The Li-release capacity of titanium dioxidedescribed in JP-A 2008-34368 (KOKAI) is 200 mAh/g or less, which islower than that of the titanium oxide compound of this embodiment.Further, in JP-A 2008-117625 (KOKAI), the heating temperature is 320°C., which is a relatively low temperature, whereas the heating time is20 hours. The initial Li-release capacity of titanium dioxide in thisJP-A 2008-117625 (KOKAI) is about 160 mAh/g, which is significantlylower.

As mentioned above, titanium dioxide synthesized by a conventionalmethod is deteriorated in electrode performance. It is understood fromthe viewpoint of electrode performance that in these conventionalmethods, the proton-exchanged compound is excessively heated.

Therefore, a titanium oxide compound which is superior in electrodeperformance and has a TiO₂(B) structure in which the plane spacing d₀₀₁is 6.22 Å or more, can be obtained by appropriately selecting theheating conditions of the proton-exchanged compound according to theembodiment.

(Third Embodiment)

In the Third embodiment, a nonaqueous electrolyte battery is provided.The nonaqueous electrolyte battery comprises a positive electrode, anegative electrode, a nonaqueous electrolyte, a separator and acontainer. In this embodiment, the negative electrode comprises anegative electrode active material according to the first embodiment.

Hereinafter, the positive electrode, negative electrode, nonaqueouselectrolyte, separator, and container will be explained in detail.

1) Positive Electrode

The positive electrode comprises a current collector and a positiveelectrode layer (namely, positive electrode active material-containinglayer). The positive electrode layer is formed on one or both surfacesof the current collector and contains a positive electrode activematerial, and optionally, a conductive agent and a binder.

Examples of the positive electrode active material include oxides andsulfides. Specific examples of the positive electrode active materialinclude manganese dioxide (MnO₂), iron oxide, copper oxide, and nickeloxide impregnated with lithium, lithium-manganese composite oxide (suchas Li_(x)Mn₂O₄ or Li_(x)MnO₂), lithium-nickel composite oxide (such asLi_(x)NiO₂), lithium-cobalt composite oxide (such as Li_(x)CoO₂),lithium-nickel-cobalt composite oxide (such as LiNi_(1-y)Co_(y)O₂),lithium-manganese-cobalt composite oxide (such asLi_(x)Mn_(y)Co_(1-y)O₂), lithium-manganese-nickel composite oxide havinga spinel structure (Li_(x)Mn_(2-y)Ni_(y)O₄), lithium-phosphorous oxidehaving an olivine structure (such as Li_(x)FePO₄,Li_(x)Fe_(1-y)Mn_(y)PO₄ and Li_(x)CoPO₄), iron sulfate [Fe₂(SO₄)₃],vanadium oxide (such as V₂O₅) and lithium-nickel-cobalt-manganesecomposite oxide. Here, x and y satisfy the following equations: 0≦x≦1and 0≦y≦1.

Preferable examples of the positive electrode active material includelithium-manganese composite oxide (Li_(x)Mn₂O₄), lithium-nickelcomposite oxide (Li_(x)NiO₂), lithium-cobalt composite oxide(Li_(x)CoO₂), lithium-nickel-cobalt composite oxide(LiNi_(1-y)Co_(y)O₂), lithium-manganese-nickel composite oxide having aspinel structure (Li_(x)Mn_(2-y)Ni_(y)O₄), lithium-manganese-cobaltcomposite oxide (Li_(x)Mn_(y)Co_(1-y)O₂), Lithium-iron phosphate(Li_(x)FePO₄) and lithium-nickel-cobalt-manganese composite oxide. Theseactive materials make it possible to obtain a high positive electrodevoltage. Here, x and y satisfy the following equations: 0≦x≦1 and 0≦y≦1.

When a nonaqueous electrolyte uses a cold molten salt, lithium-ironphosphate, Li_(x)VPO₄F, lithium-manganese composite oxide,lithium-nickel composite oxide and lithium-nickel-cobalt composite oxideare preferably used from the viewpoint of cycle life. This is becausethe use of these oxides brings about less reactivity between thepositive electrode active material and the cold molten salt.

The primary particle diameter of the positive electrode active materialis preferably 100 nm to 1 μm. A positive electrode active materialhaving a primary particle diameter of 100 nm or more is easily handledin industrial production. A positive electrode active material having aprimary particle diameter of 1 μm or less enables lithium ions todiffuse smoothly in solid.

The specific surface area of the positive electrode active material ispreferably 0.1 m²/g to 10 m²/g. A positive electrode active materialhaving a specific surface area of 0.1 m²/g or more can secure lithiumion-absorption and release sites sufficiently. A positive electrodeactive material having a specific surface area of 10 m²/g or less iseasily handled in industrial production and ensures a goodcharge-discharge cycle performance.

Examples of the binder for binding the positive electrode activematerial with the current collector include polytetrafluoroethylene(PTFE), polyvinylidene fluoride (PVdF) and fluoro-rubber.

The conductive agent is formulated as required to improve the currentcollecting ability and to reduce the contact resistance with the currentcollector. Examples of the conductive agent include carbonaceousmaterials such as acetylene black, carbon black and graphite.

The ratios of the positive electrode active material and binder arepreferably 80% by mass to 98% by mass and 2% by mass to 20% by massrespectively. When the amount of the binder is 2% by mass or more,satisfactory electrode strength is obtained. Further, when the amount ofthe binder is 20% by mass or less, the amount of the insulating materialof the electrode can be reduced, leading to reduced internal resistance.

When the conductive agent is added, its amount is designed to be 3% bymass or more to obtain the effect of its addition. When its amount isdesigned to be 15% by mass or less, on the other hand, the decompositionof the nonaqueous electrolyte on the surface of the conductive agent canbe reduced even when the battery is stored at high temperatures.

The positive electrode can be manufactured by, for example, suspendingthe positive electrode active material and binder and the conductiveagent if necessary, in an appropriate solvent to prepare a slurry, byapplying this slurry to the surface of the current collector and dryingto form a positive electrode layer, which is then pressed.

Alternatively, the positive electrode can be manufactured by mixing thepositive electrode active material and binder, and the conductive agentif necessary, forming the mixture into a pellet. The pellet can be usedas the positive electrode layer.

The current collector is preferably an aluminum foil or an aluminumalloy foil.

The thickness of the aluminum foil or aluminum alloy foil is preferably5 μm to 20 μm and more preferably 15 μm or less. The purity of thealuminum foil is preferably 99% by mass or more. The aluminum alloy ispreferably an alloy containing elements such as magnesium, zinc andsilicon. The content of transition metals such as iron, copper, nickeland chromium contained in the aluminum foil or aluminum alloy foil ispreferably designed to be 1% by mass or less.

2) Negative Electrode

The negative electrode comprises a current collector and a negativeelectrode layer (namely, negative electrode active material-containinglayer). The negative electrode layer is formed on one or both surfacesof the current collector and contains a negative electrode activematerial, a conductive agent and a binder. In the negative electrodelayer, the binder is filled in clearances between the dispersed negativeelectrode active materials. A conductive agent is formulated in thenegative electrode layer to improve the current collecting performanceand to restrain the contact resistance with the current collector.

The negative electrode active material comprises the titanium oxidecompound according to the first embodiment. The titanium oxide compoundhas a crystal structure of a monoclinic titanium dioxide and satisfiesthe following equation (I):d ₀₀₁≧6.22 Å(0.622 nm)  (I)

wherein d₀₀₁ is the spacing of the (001) plane which is measured by thepowder X-ray diffraction method using a Cu-Kα radiation source. Thistitanium oxide compound preferably has a peak intensity ratioI(200)/I(001) of 0.5 or less.

The above titanium oxide compound according to the first embodiment maybe used solely as the negative electrode active material. Alternatively,it may be used together with other compound as the negative electrodeactive material. The examples of the other compound which may be used asthe negative electrode active material include titanium dioxide havingan anatase structure TiO₂, lithium titanate having a rhamsdelitestructure Li₂Ti₃O₇ and lithium titanate having a spinel structureLi₄Ti₅O₁₂. These compounds are preferable because they have a specificgravity close to titanium oxide compound according to the firstembodiment and are easily mixed and dispersed.

Examples of the conductive agent include carbonaceous materials such asacetylene black, carbon black and graphite.

Examples of the binder include a polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVdF), fluoro-rubber and styrene-butadienerubber.

The binder is preferably formulated in an amount range from 2% by massto 30% by mass in the negative electrode layer. When the amount of thebinder is 2% by mass or more, the binding force between the negativeelectrode layer and the current collector is satisfactory and excellentcycle performances can be expected. On the other hand, the amount of thebinder is preferably 30% by mass or less from the viewpoint of highcapacity. The conductive agent is also preferably formulated in anamount of 30% by mass or less in the negative electrode layer.

For the current collector, materials which are electrochemically stableat the lithium absorption and release potential of the negativeelectrode active material are used. The current collector is preferablymade of copper, nickel, stainless or aluminum. The thickness of thecurrent collector is preferably 5 to 20 μm. A current collector havingsuch a thickness can keep the balance between the strength of thenegative electrode and light-weight performances.

The negative electrode can be manufactured by, for example, suspendingthe negative electrode active material, binder and conductive agent in ausual solvent to prepare a slurry, by applying this slurry to thesurface of the current collector and by drying to form a negativeelectrode layer, which is then pressed.

Alternatively, the negative electrode can be manufactured by mixing thenegative electrode active material, binder, and the conductive agent,forming the mixture into a pellet. The pellet can be used as thenegative electrode layer.

3) Nonaqueous Electrolyte

Examples of the nonaqueous electrolyte include a liquid nonaqueouselectrolyte and a gel-like nonaqueous electrolyte. The liquid nonaqueouselectrolyte is prepared by dissolving an electrolyte in an organicsolvent. The gel-like nonaqueous electrolyte is prepared by forming acomposite of a liquid electrolyte and a polymer material.

The liquid nonaqueous electrolyte is dissolved in an organic solvent ina concentration of 0.5 mol/L to 2.5 mol/L.

Examples of the electrolyte include lithium salts such as lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumtetrafluoroborate (LiBF₄), hexafluoro arsenic lithium (LiAsF₆), lithiumtrifluoromethasulfonate (LiCF₃SO₃), bistrifluoromethylsulfonylimidelithium [LiN(CF₃SO₂)₂], or mixtures of these compounds. The electrolyteis preferably one which is scarcely oxidized even at a high potentialand LiPF₆ is most preferable.

Examples of the organic solvent include cyclic carbonates such aspropylene carbonate (PC), ethylene carbonate (EC) and vinylenecarbonate, chain carbonates such as diethyl carbonate (DEC), dimethylcarbonate (DMC) and methylethyl carbonate (MEC), cyclic ethers such astetrahydrofuran (THF), 2-methyltetrahydrofuran (2MeTHF) and dioxolan(DOX), chain ethers such as dimethoxyethane (DME) and diethoethane(DEE), γ-butyrolactone (GBL), acetonitrile (AN) and sulfolan (SL). Theseorganic solvents may be used either singly or in combinations of two ormore.

Examples of the polymer material include a polyvinylidene fluoride(PVdF), polyacrylonitrile (PAN) and polyethylene oxide (PEO).

A cold molten salt (ionic melt) containing lithium ions, polymer solidelectrolyte, inorganic solid electrolyte and the like may also be usedas the nonaqueous electrolyte.

The cold molten salt (ionic melt) means compounds which may exist in aliquid state at normal temperature (15 to 25° C.) among organic saltsconstituted of combinations of organic cations and anions. The coldmolten salts include those which singly exist in a liquid state, thosewhich are put into a liquid state when mixed with an electrolyte andthose which are put into a liquid state when dissolved in an organicsolvent. Generally, the melting point of the cold molten salt used in anonaqueous electrolyte battery is 25° C. or less. Further, the organiccation generally has a quaternary ammonium skeleton.

The polymer solid electrolyte is prepared by dissolving an electrolytein a polymer material and by solidifying the electrolyte mixture.

The inorganic solid electrolyte is a solid material having lithiumion-conductivity.

4) Separator

The separator may be formed of a porous film comprising a polyethylene,polypropylene, cellulose or polyvinylidene fluoride (PVdF), or syntheticresin nonwoven fabric. Among these materials, a porous film formed of apolyethylene or polypropylene melts at a fixed temperature, making itpossible to shut off current and is therefore preferable from theviewpoint of improving safety.

5) Container

A container made of a laminate film or a metal container may be used.The laminate film preferably has a thickness of 0.5 mm or less. Themetal container preferably has a thickness of 1.0 mm or less. Thethickness of the laminate film is more preferably 0.2 mm or less. Themetal container has a thickness of, more preferably, 0.5 mm or less andmost preferably 0.2 mm or less.

Examples of the shape of the container include a flat type (thin type),angular type, cylinder type, coin type and button type. The containerhaving a size corresponding to the dimensions of a battery are used. Forexample, containers for small-sized batteries to be mounted on portableelectronic devices and containers for large-sized batteries to bemounted on, for example, two- to four-wheel vehicles are used.

As the laminate film, a multilayer film prepared by interposing a metallayer between resin layers may be used. The metal layer is preferablyformed of an aluminum foil or aluminum alloy foil to reduce the weightof the battery. For example, polymer materials such as polypropylene(PP), polyethylene (PE), nylon and polyethylene terephthalate (PET) maybe used for the resin layer. The laminate film can be molded into adesired shape by sealing through thermal fusion.

The metal container is made of aluminum, an aluminum alloy or the like.The aluminum alloy is preferably an alloy containing one or moreelements selected from magnesium, zinc, and silicon. When the alloycontains transition metals such as iron, copper, nickel and chromium,the amount of the transition metals is preferably 1 mass % or less. Thissignificantly improves the long-term reliability under ahigh-temperature environment and heat dissipation performances.

6) Nonaqueous Electrolyte Battery

Next, the nonaqueous electrolyte battery according to the thirdembodiment will be explained in more detail with reference to thedrawings. The same reference numeral is attached to a structure commonto the embodiments and duplicated explanations are omitted here. Eachdrawing is a typical view for explaining the embodiment and forpromoting the understanding of the embodiment. Though there are partsdifferent from an actual battery in shape, dimension and ratio, thesestructural designs may be properly changed taking the followingexplanations and known technologies into consideration.

FIG. 2 is a sectional view of a flat type nonaqueous electrolytesecondary battery. FIG. 3 is an enlarged sectional view of the A-part ofFIG. 2.

A flat type coil electrode group 1 is accommodated in a baggy container2 made of a laminate film obtained by interposing an aluminum foilbetween two resin layers. The coil electrode groups 1 having a flat formare formed by spirally coiling a laminate obtained by laminating anegative electrode 3, a separator 4, a positive electrode 5 and aseparator 4 in this order from the outside and by press-molding thecoiled laminate. The outermost negative electrode 3 has a structure inwhich a negative electrode layer 3 b is formed on one inside surface ofa negative electrode current collector 3 a as shown in FIG. 3. Othernegative electrodes 3 each have a structure in which a negativeelectrode layer 3 b is formed on each surface of the current collector 3a. An active material comprised in the negative electrode layer 3 bcontains the negative electrode active material as mentioned in thefirst embodiment.

In the positive electrode 5, a positive electrode layer 5 b is formed oneach surface of a positive electrode current collector 5 a.

In the vicinity of the outer peripheral end of the coil electrode group1, a negative electrode terminal 6 is connected to the negativeelectrode current collector 3 a of the outermost negative electrode 3and a positive electrode terminal 7 is connected to the positiveelectrode current collector 5 a of the inside positive electrode 5. Thenegative electrode terminal 6 and positive electrode terminal 7 areexternally extended from an opening part of the baggy container 2. Aliquid nonaqueous electrolyte is injected from the opening part of thebaggy container 2. The opening part of the baggy container 2 is closedby heat sealing with the negative electrode terminal 6 and positiveelectrode terminal 7 extended out of the opening part to therebyperfectly seal the coil electrode group 1 and liquid nonaqueouselectrolyte.

The negative electrode terminal 6 is made of, for example, a materialhaving electric stability and conductivity at the Li-absorption andrelease potential of the negative electrode active material.Specifically, examples of these materials include copper, nickel,stainless and aluminum. The negative electrode terminal is preferablymade of the same material as the negative electrode current collector toreduce the contact resistance with the negative electrode currentcollector.

The positive electrode terminal 7 is made of, for example, a materialhaving electric stability and conductivity in a potential range from,preferably, 3 V to 5 V relative to a lithium ion metal. Specificexamples of these materials include aluminum alloys containing elementssuch as Mg, Ti, Zn, Mn, Fe, Cu and Si, and aluminum. The positiveelectrode terminal is preferably made of the same material as thepositive electrode current collector to reduce the contact resistancewith the positive electrode current collector.

The nonaqueous electrolyte secondary battery according to the thirdembodiment is not limited to the structure shown in FIG. 2 and FIG. 3and may have a structure as shown in, for example, FIG. 4 and FIG. 5.FIG. 4 is a partly broken perspective view typically showing anotherflat type nonaqueous secondary battery, and FIG. 5 is an enlargedsectional view of the B part of FIG. 4.

A laminate type electrode group 11 is accommodated in a container 12made of a laminate film obtained by interposing a metal layer betweentwo resin layer. The laminate type electrode group 11 has a structure inwhich a positive electrode 13 and a negative electrode 14 are, as shownin FIG. 5, alternately laminated with a separator 15 interposedtherebetween. The positive electrodes 13 exist in plural, each beingprovided with a current collector 13 a and a positive electrode activematerial-containing layer 13 b carried on each side of the currentcollector 13 a. The negative electrodes 14 exist in plural, each beingprovided with a current collector 14 a and a negative electrode activematerial-containing layer 14 b carried on each side of the currentcollector 14 a. One side of the current collector 14 a of each negativeelectrode 14 is projected from the positive electrode 13. The projectedcurrent collector 14 a is electrically connected to a band-shapednegative electrode terminal 16. The end of the band-shaped negativeelectrode terminal 16 is externally drawn out of the package member 11.Further, of the current collector 13 a of the positive electrode 13, theside positioned opposite to the projected side of the current collector14 a is projected from the negative electrode 14. The current collector13 a projected from the negative electrode 14 is electrically connectedto a band-shaped positive electrode terminal 17. The end of theband-shaped positive electrode terminal 17 is positioned opposite to thenegative electrode terminal 16 and drawn out of the side of the packagemember 11.

According to the third embodiment, the nonaqueous electrolyte battery isprovided with the negative electrode comprising the negative electrodeactive material according to the first embodiment. Therefore, anonaqueous electrolyte battery having a high effective capacity andexcellent repeat charge/discharge performances can be provided.

(Fourth Embodiment)

Next, a battery pack according to a fourth embodiment will be explainedwith reference to the drawings. The battery pack comprises one or two ormore of the nonaqueous electrolyte batteries (unit cells) according tothe third embodiment. When the battery pack includes two or more unitcells, these unit cells are disposed in such a manner that they areelectrically connected in series or in parallel.

Such a battery pack will be explained in detail with reference to FIG. 6and FIG. 7. A flat type battery as shown in FIG. 2 may be used as theunit cell 21.

A plurality of unit cells 21 are laminated such that the externallyextended negative electrode terminal 6 and positive electrode terminal 7are arranged in the same direction and fastened with an adhesive tape 22to thereby constitute a battery module 23. These unit cells 21 areelectrically connected in series as shown in FIG. 7.

A printed wiring board 24 is disposed opposite to the side surface ofthe unit cell 21 from which the negative electrode terminal 6 andpositive electrode terminal 7 are extended. As shown in FIG. 7, athermistor 25, a protective circuit 26 and an energizing terminal 27connected to external devices are mounted on the printed wiring board24. An insulating plate (not shown) is attached to the surface of theprotective circuit substrate 24 facing the battery module 23 to avoidunnecessary connection with the wiring of the battery module 23.

A positive electrode side lead 28 is connected to the positive electrodeterminal 7 positioned on the lowermost layer of the battery module 23and one end of the positive electrode side lead 28 is inserted into andelectrically connected to a positive electrode side connector 29 of theprinted wiring board 24. A negative electrode side lead 30 is connectedto the negative electrode terminal 6 positioned on the uppermost layerof the battery module 23 and one end of the negative electrode side lead30 is inserted into and electrically connected to a negative electrodeside connector 31 of the printed wiring board 24. These connectors 29and 31 are connected to the protective circuit 26 through wirings 32 and33 formed on the printed wiring board 24.

The thermistor 25 is used to detects the temperature of the unit cell 21and the detected signals are transmitted to the protective circuit 26.The protective circuit 26 can shut off a plus side wiring 34 a and minusside wiring 34 b between the protective circuit 26 and the energizingterminal 27 connected to external devices in a predetermined condition.The predetermined condition means, for example, the case where thetemperature detected by the thermistor 25 is a predetermined one orhigher. Also, the predetermined condition means, for example, the caseof detecting overcharge, overdischarge and over-current of the unit cell21. The detections of this overcharge and the like are made forindividual unit cells 21 or whole unit cells 21. When individual unitcells 21 are detected, either the voltage of the battery may be detectedor the potential of the positive electrode or negative electrode may bedetected. In the latter case, a lithium electrode used as a referenceelectrode is inserted between individual unit cells 21. In the case ofFIG. 6 and FIG. 7, a wiring 35 for detecting voltage is connected toeach unit cell 21 and the detected signals are transmitted to theprotective circuit 26 through these wirings 35.

A protective sheet 36 made of a rubber or resin is disposed on each ofthe three side surfaces of the battery module 23 excluding the sidesurface from which the positive electrode terminal 7 and negativeelectrode terminal 6 are projected.

The battery module 23 is accommodated in a receiving container 37together with each protective sheet 36 and printed wiring board 24.Specifically, the protective sheet 36 is disposed on each inside surfacein the direction of the long side and on one of the inside surfaces inthe direction of the short side of the receiving container 37, and theprinted wiring board 24 is disposed on the other inside surface in thedirection of the short side. The battery module 23 is positioned in aspace enclosed by the protective sheet 36 and the printed wiring board24. A lid 38 is attached to the upper surface of the receiving container37.

Here, a thermally contracting tape may be used in place of the adhesivetape 22 to secure the battery module 23. In this case, after theprotective sheet is disposed on both sides of the battery module and thethermally contracting tubes are wound around the battery module; thethermally contracting tape is contracted by heating to fasten thebattery module.

The structure in which the unit cells 21 are connected in series isshown in FIG. 6 and FIG. 7. However, these unit cells may be connectedin parallel to increase the capacity of the battery. The assembledbattery packs may be further connected in series or in parallel.

Also, the structure of the battery pack is appropriately changedaccording to its use. The battery pack is preferably used inapplications exhibiting excellent cycle performances when a largecurrent is extracted. Specific examples of these applications includepower sources for digital cameras, and power sources mounted on vehiclessuch as two- to four-wheel vehicles hybrid electric cars, two- tofour-wheel electric cars and assist bicycles. The battery pack ispreferably used for power sources mounted on vehicles.

EXAMPLES

The embodiments will be explained in more detail by way of examples.However, the embodiments are not limited to these examples. Theidentification of the crystal phase and the estimation of the crystalstructure as to the product obtained in the synthesis were made by thepowder X-ray diffraction method using Cu-Kα radiation. The measurementof specific surface area of the products was made by the BET method.Further, the composition of the products was analyzed by the ICP methodto confirm that a target product was obtained.

Synthetic Example 1 Synthesis of a Titanium Oxide Compound by UsingK₂Ti₄O₉ as Starting Material

A commercially available K₂Ti₄O₉ reagent was used as a starting materialto synthesize a proton titanate compound H₂Ti₄O₉. First, a K₂Ti₄O₉powder was washed with distilled water to remove impurities. Then, 5 gof the K₂Ti₄O₉ powder was poured into a zirconia pot having an internalvolume of 100 cm³, to which was then added zirconia balls 10 mm indiameter to fill about ⅓ of the volume of the pot. This pot was rotatedat 800 rpm for 2 hours to grind K₂Ti₄O₉ into particles having an averageparticle diameter of about 5 μm.

Then, the ground K₂Ti₄O₉ powder was added in a 1 M hydrochloric acidsolution and stirred at 25° C. for 72 hours. The 1 M hydrochloric acidwas replaced every 24 hours. Potassium ions were thereby exchanged forprotons to obtain a proton-exchanged compound H₂Ti₄O₉.

The obtained suspension had good dispersibility and could not beseparated by filtration. Therefore, the proton-exchanged compound wasisolated from a solvent by a centrifuge. A powder of the obtainedproton-exchanged compound was washed with distilled water until the pHof the washing solution was 6 to 7.

Then, the proton-exchanged compound was heated for 3 hours. Thetemperature of the proton-exchanged compound was varied in increments of25° C. in a temperature range from 300 to 400° C. to determine anappropriate heating condition. The heating temperature was set asfollows: Synthetic Example 1-1: 300° C., Synthetic Example 1-2: 325° C.,Synthetic Example 1-3: 350° C., Synthetic Example 1-4: 375° C. andSynthetic Example 1-5: 400° C. In order to obtain an exact heat history,the sample was placed in an electric furnace preheated to the settemperatures. After being heated, the sample was taken out of thefurnace and quenched rapidly in air. The heated sample was dried at 80°C. in a vacuum for 12 hours to obtain titanium dioxide having a TiO₂(B)structure.

Further, in order to change the specific surface area, Synthetic Example1-3 was mechanically ground to make a sample as Synthetic Example 1-6.Synthetic Example 1-6 was titanium dioxide having a TiO₂(B) structureand a higher specific surface area than Synthetic Example 1-3.

(Powder X-Ray Diffraction Measurement)

Each titanium dioxide obtained in Synthetic Examples 1-1 to 1-6 wasmeasured by the powder X-ray diffraction method to obtain the planespacing (d₀₀₁) and peak intensity ratio I(200)/I(001). The results areshown in Table 1. The value of d₀₀₁ of each titanium dioxide was 6.215 Åto 6.238 Å (error: within 0.001 Å). The peak intensity ratioI(200)/I(001) was 0 to 0.65.

(Production of an Electrochemical Measuring Cell)

The titanium dioxide synthesized above was used to manufacture anelectrode. A polytetrafluoroethylene was mixed as a binder in eachtitanium dioxide powder obtained in Synthetic Examples 1-1 to 1-6 in anamount of 10% by mass based on the total mass of the electrode. Theobtained mixture was molded to manufacture an electrode.

Ethylene carbonate and diethyl carbonate were mixed in a ratio by volumeof 1:1 to prepare a mixed solvent. 1 M lithium perchlorate was added tothe mixed solvent to prepare an electrolytic solution.

The above titanium dioxide electrode, a counter electrode and theelectrolytic solution were used to manufacture a measuring cell. As thecounter electrode, a metal lithium foil was used.

Since a lithium metal is used as the counter electrode in this measuringcell, the electrode potential of the titanium dioxide electrode is noblerelative to the counter electrode. Thus, the titanium dioxide electrodewas worked as the positive electrode. However, the titanium dioxideelectrode may be used as a negative electrode if it is combined with apositive electrode active material. In the measuring cell, thedirections of charge and discharge are opposite to those of a batteryusing titanium dioxide as the negative electrode active material. Inorder to avoid confusion here, the direction in which lithium ions areinserted into the titanium dioxide electrode is called a chargedirection and the direction in which lithium ions are released from thetitanium dioxide electrode is called a discharge direction.

(Evaluation of Charge/Discharge Capacity)

The charge/discharge capacity of each measuring cell of SyntheticExamples 1-1 to 1-5 was measured. In the measurement, the measuring cellwas made to charge and discharge in the condition of a potential rangefrom 1.0 V to 3.0 V based on a metal lithium electrode, acharge/discharge current of 0.05 mA/cm² and ambient temperature.

(Evaluation of Repeat Discharge Performances)

Using each of the measuring cells obtained in Synthetic Examples 1-1 to1-5, a charge-discharge operation was repeated for 50 cycles (chargeoperation and discharge operation=one cycle) to examine the dischargecapacity retention ratio. The measurement was made in the condition of apotential range from 1.0 V to 3.0 V based on metal lithium electrode, acharge/discharge current of 0.05 mA/cm² and ambient temperature.

The capacity retention ratio based on the first cycle discharge capacitywas calculated. In the case, the first cycle discharge capacity when thecharge/discharge current was 0.05 mA/cm² was considered 100%.

(Results)

The results are shown in Table 1. Each of Synthetic Examples 1-1 to 1-4had d₀₀₁ of 6.22 Å or more and a peak intensity ratio I(200)/I(001) was0.5 or less. Synthetic Example 1-5 had d₀₀₁ less than 6.22 Å and a peakintensity ratio I(200)/I(001) exceeded 0.5. This fact shows that inSynthetic Example 1, titanium dioxide having a TiO₂(B) structure andalso having d₀₀₁ of 6.22 Å or more and a peak intensity ratioI(200)/I(001) of 0.5 or less is obtained when the heating temperature is375° C. or less.

Synthetic Examples 1-1 to 1-4 and 1-6 respectively had a higher firstcycle discharge capacity than Synthetic Example 1-5. This fact showsthat titanium dioxide having a TiO₂(B) structure and also having d₀₀₁ of6.22 Å or more and a peak intensity ratio I(200)/I(001) of 0.5 or lesshas a high discharge capacity.

Synthetic Examples 1-3 and 1-6 in which the heating temperature is 350°C. have a higher first cycle discharge capacity and capacity retentionratio than others, showing that the optimal heating temperature inSynthetic Example 1 is close to 350° C.

Synthetic Example 1-1 had a low capacity retention ratio. This isconsidered to be because the TiO₂(B) structure of Synthetic Example 1-1has less crystallinity.

The above Synthetic Examples 1-3 and 1-6 were renamed as Example 1 andExample 2 respectively. A powder X-ray diffraction diagram using a Cu-Kαas the radiation source in this Example 1 is shown in FIG. 8. In themeasurement of X-ray diffraction, the sample was ground until theaverage particle diameter was about 5 μm and then filled in the holderpart of the glass sample plate in the same method as above. The samplewas measured in the following conditions: scanning speed: 3 deg/min,step width: 0.2 deg, tube voltage: 40 kV and tube current: 20 mA.Further, the specific surface area of Example 1 was measured by the BETmethod. The results are shown in Table 3. Titanium dioxide of Example 1had a specific surface area of 6.3 m²/g. Further, the ground titaniumdioxide of Example 2 had a specific surface area of 21.8 m²/g. AlthoughExample 1 substantially differed from Example 2 only in specific surfacearea, Example 1 had a higher initial capacity. This is considered to bebecause Example 1 had a lower specific surface area so that sidereactions caused by the contact with the electrolytic solution islimited, resulting in a low resistance overvoltage. Hereinafter, Example1 will be explained as a typical example.

TABLE 1 Heating First cycle Discharge capacity Synthetic temperaturedischarge capacity retention ratio Example (° C.) d₀₀₁ (Å) I(200)/I(001)(mAh/g) after 50 cycles (%) 1-1 300 6.234 0 221 69 1-2 325 6.237 0.35225 85 1-3 350 6.239 0.37 238 98 1-4 375 6.221 0.47 216 95 1-5 400 6.2150.65 198 95 1-6 350 6.238 0.39 228 97

Synthetic Example 2 Synthesis of a Titanium Oxide Compound UsingNa₂Ti₃O₇ as Starting Material

A commercially available Na₂Ti₃O₇ reagent was used as a startingmaterial to synthesize a proton titanate compound H₂Ti₃O₇. First, apowder of Na₂Ti₃O₇ was washed with distilled water to remove impurities.Next, 5 g of the Na₂Ti₃O₇ powder was poured into a zirconia pot havingan internal volume of 100 cm³, to which was then added zirconia balls 10mm in diameter to fill about ⅓ of the volume of the pot. This pot wasrotated at 800 rpm for 2 hours to grind Na₂Ti₃O₇.

The sintering temperature of sodium titanate is lower than that ofpotassium titanate. This enables sodium titanate to have a smalleraverage particle diameter. Therefore, the powder was ground until theaverage particle diameter was decreased to about 1 μm.

Then, the ground Na₂Ti₃O₇ powder was added in a 1 M hydrochloric acidsolution and stirred at 25° C. for 48 hours. The 1 M hydrochloric acidwas replaced every 24 hours. Sodium ions were thereby exchanged forprotons to obtain a proton-exchanged compound H₂Ti₃O₇.

The obtained suspension had good dispersibility and could not beseparated by filtration. Therefore, the proton-exchanged compound wasisolated from a solvent by using a centrifuge. A powder of the obtainedproton-exchanged compound was washed with distilled water until the pHof the washing solution was 6 to 7.

Then, the proton-exchanged compound was heated for 3 hours. Thetemperature of the proton-exchanged compound was varied in increments of25° C. in a temperature range from 300 to 400° C. to determine anappropriate heating condition. The heating temperature was set asfollows: Synthetic Example 2-1: 300° C., Synthetic Example 2-2: 325° C.,Synthetic Example 2-3: 350° C., Synthetic Example 2-4: 375° C. andSynthetic Example 2-5: 400° C. In order to obtain an exact heat history,the sample was placed in an electric furnace preheated to a settemperature. After being heated, the sample was taken out of the furnaceand quenched rapidly in air. The heated sample was dried at 80° C. in avacuum for 12 hours to obtain titanium dioxide having a TiO₂(B)structure.

(Powder X-Ray Diffraction Measurement)

Each titanium dioxide obtained in Synthetic Examples 2-1 to 2-5 wasmeasured by the powder X-ray diffraction method in the same manner as inSynthetic Example 1, to obtain the d₀₀₁ and peak intensity ratioI(200)/I(001). The results are shown in Table 2. The d₀₀₁ of eachtitanium dioxide obtained in Examples 2-1 to 2-5 was 6.175 Å to 6.231 Å(error: within 0.001 Å). The peak intensity ratio I(200)/I(001) was 0 to0.83.

(Production of an Electrochemical Measuring Cell)

The titanium dioxide produced above was used to manufacture anelectrochemical measuring cell in the same manner as in SyntheticExample 1.

(Evaluation of Charge/Discharge Capacity)

The charge/discharge capacity of each measuring cell of SyntheticExamples 2-1 to 2-5 was measured in the same manner as in SyntheticExample 1.

(Evaluation of Repeat Discharge Performances)

The discharge capacity retention ratio of each of the measuring cells ofSynthetic Examples 2-1 to 2-5 was examined in the same manner as inSynthetic Example 1.

(Results)

The results are shown in Table 2. Each of Synthetic Examples 2-1 to 2-3had d₀₀₁ of 6.22 Å or more and a peak intensity ratio I(200)/I(001) was0.5 or less. Synthetic Examples 2-4 and 2-5 respectively had d₀₀₁ lessthan 6.22 Å and also, a peak intensity ratio I(200)/I(001) exceeded 0.5.It is shown from this fact that in Synthetic Example 2, titanium dioxidehaving a TiO₂(B) structure and also having d₀₀₁ of 6.22 Å or more and apeak intensity ratio I(200)/I(001) of 0.5 or less is obtained when theheating temperature is 350° C. or less.

Synthetic Examples 2-1 to 2-3 respectively had a higher first cycledischarge capacity than Synthetic Examples 2-4 and 2-5. It is shown fromthis fact that titanium dioxide having a TiO₂(B) structure and alsohaving d₀₀₁ of 6.22 Å or more and a peak intensity ratio I(200)/I(001)of 0.5 or less has a high discharge capacity.

Synthetic Example 2-3 in which the heating temperature is 350° C. hasthe highest first cycle discharge capacity and capacity retention ratio,showing that the optimal heating temperature in Synthetic Example 2 isclose to 350° C.

Synthetic Example 2-1 had a low capacity retention ratio. This isconsidered to be because the TiO₂(B) structure of Synthetic Example 2-1has less crystallinity.

The above Synthetic Examples 2-3 was renamed as Example 3. A powderX-ray diffraction diagram using a Cu-Kα as the radiation source in thisExample 2 is shown in FIG. 9. The measurement of X-ray diffraction wasmade in the same manner as in Example 1. Further, the specific surfacearea of Example 2 was measured by the BET method. The results are shownin Table 3. The titanium dioxide of Example 3 had a specific surfacearea of 37.5 m²/g.

TABLE 2 Heating First cycle Discharge capacity Synthetic temperaturedischarge capacity retention ratio Example (° C.) d₀₀₁ (Å) I(200)/I(001)(mAh/g) after 50 cycles (%) 2-1 300 6.231 0 198 63 2-2 325 6.225 0.39205 79 2-3 350 6.223 0.43 226 85 2-4 375 6.193 0.55 178 87 2-5 400 6.1750.83 171 84

Comparative Example 1 Synthesis of TiO₂(B)

As Comparative Example 1, TiO₂(B) was synthesized according to thesynthetic method described in JP-A 2008-34368 (KOKAI). Potassium nitrateand titanium dioxide having an anatase structure were mixed in apredetermined ratio and the mixture was heated at 1000° C. for 24 hoursto obtain a compound K₂Ti₄O₉. Next, this compound was poured into anaqueous 1 M nitric acid solution and stirred at ambient temperature for12 hours. The obtained powder was washed several times with distilledwater and heated at 400° C. for 3 hours to obtain TiO₂(B).

(Powder X-Ray Diffraction Measurement)

TiO₂(B) of Comparative Example 1 was measured by the powder X-raydiffraction method in the same manner as in Synthetic Example 1 toobtain the d₀₀₁ and peak intensity ratio I(200)/I(001). The results areshown in Table 3. The d₀₀₁ was 6.212 Å (error: within 0.001 Å). The peakintensity ratio I(200)/I(001) was 0.67.

(Measurement of Specific Surface Area)

The specific surface area was measured by the BET method. TiO₂(B) ofComparative Example 1 had a specific surface area of 8.3 m²/g.

(Production of an Electrochemical Measuring Cell)

An electrochemical measuring cell was manufactured using TiO₂(B)generated above.

The cell was manufactured in the same manner as in Synthetic Example 1except that acetylene black was used as a conductive adjuvant in anamount of 30% by mass based on the total mass of the electrode.

(Evaluation of Charge/Discharge Capacity)

The charge/discharge capacity of the measuring cell of ComparativeExample 1 was measured in the same manner as in Synthetic Example 1.

(Evaluation of Repeat Discharge Performances)

The discharge capacity retention ratio of the measuring cell ofComparative Example 1 was examined in the same manner as in SyntheticExample 1.

(Results)

The results are shown in Table 3.

Comparative Example 2 Synthesis of TiO₂(B)

As Comparative Example 2, TiO₂(B) was synthesized according to thesynthetic method described in JP-A 2008-117625 (KOKAI). A sodiumcarbonate powder and titanium dioxide powder which were high-purityreagent were weighed and mixed such that the ratio by mol of Na:Ti=2:3.The resulting mixture was heated at 800° C. for 20 hours and thisheating operation was repeated twice. The resulting Na₂Ti₃O₇polycrystals were dipped in a 0.5 M hydrochloric acid solution, whichwas then kept at ambient temperature for 5 days to carry out protonexchange treatment. Thereafter, the resulting product was washed withwater and dried at 120° C. under vacuum for 24 hours to obtain aproton-exchanged compound of H₂Ti₃O₇ polycrystals.

Next, the obtained H₂Ti₃O₇ polycrystals were heated at 320° C. in airfor 20 hours to obtain TiO₂(B).

(Powder X-Ray Diffraction Measurement)

TiO₂(B) of Comparative Example 2 was measured by the powder X-raydiffraction method in the same manner as in Synthetic Example 1 toobtain the d₀₀₁ and peak intensity ratio I(200)/I(001). The results areshown in Table 3. The d₀₀₁ was 6.217 Å (error: within 0.001 Å). The peakintensity ratio I(200)/I(001) was 0.65.

(Measurement of Specific Surface Area)

The specific surface area was measured by the BET method. TiO₂(B) ofComparative Example 2 had a specific surface area of 160.5 m²/g.

(Production of an Electrochemical Measuring Cell)

An electrochemical measuring cell was manufactured using TiO₂(B)generated above in the same manner as in Synthetic Example 1.

(Evaluation of Charge/Discharge Capacity)

The charge/discharge capacity of the measuring cell of ComparativeExample 2 was measured in the same manner as in Synthetic Example 1.

(Evaluation of Repeat Discharge Performances)

The discharge capacity retention ratio of the measuring cell ofComparative Example 2 was examined in the same manner as in SyntheticExample 1.

(Results)

The results are shown in Table 3.

TABLE 3 First cycle Discharge capacity Specific discharge capacityretention ratio surface area d₀₀₁ (Å) I(200)/I(001) (mAh/g) after 50cycles (%) (m²/g) Example 1 6.239 0.37 238 98 6.3 Example 2 6.238 0.39228 97 21.8 Example 3 6.223 0.43 226 85 37.5 Comparative 6.212 0.67 19580 8.3 Example 1 Comparative 6.217 0.65 162 78 160.5 Example 2<Evaluation>

FIG. 10 shows charge/discharge curves of Examples 1 and 3 andComparative Examples 1. As is understood from FIG. 10 and Table 3, it isshown that each first cycle discharge capacity of Examples 1 and 3 washigher by 15% to 40% than that of each of Comparative Examples 1 and 2obtained by the conventionally known method.

Comparative Examples 1 and 2 each had a lower capacity retention ratiothan each of Examples 1 to 3. This suggests that many lithium ions aretrapped by a crystal in TiO₂(B) obtained in each of Comparative Examples1 and 2. It is considered that the crystal structures of ComparativeExamples 1 and 2 are not most suitable. This is also evidenced by thefact that the d₀₀₁ of each of Comparative Examples 1 and 2 is less than6.22 Å and the peak intensity ratio I(200)/I(001) exceeds 0.5. Further,the fact that TiO₂(B) obtained in each of Comparative Examples 1 and 2has a peak intensity ratio I(200)/I(001) exceeding 0.5 suggests that thecrystallinity becomes too high due to excess heating and also, a traceamount of impurity phase is generated.

The above fact showed that the titanium oxide compound according to theembodiments has a higher discharge capacity and provides more excellentrepeat charge/discharge performances than the titanium dioxidesynthesized according to the conventionally known method, and therefore,enables stable charge/discharge operations.

<Relation Between the d₀₀₁ and Discharge Capacity>

FIG. 11 shows the relation between the first cycle discharge capacityand the d₀₀₁ value of TiO₂(B) synthesized in each of Synthetic Examples1 and 2 and Comparative Examples 1 and 2. As is clear from this FIG. 11,it is found that a first cycle discharge capacity as high as 200 mAh/gor more is shown in a d₀₀₁ range of 6.22 Å or more.

Therefore, it is shown that the titanium oxide compound which issynthesized by the method according to the embodiment and has a d₀₀₁value of 6.22 Å or more has a larger discharge capacity than thetitanium dioxide synthesized by a conventional synthetic method.

Although the heating time in the above Synthetic Examples was 3 hours,the heating time is not limited to this and may be properly changed. Inthe case where, for example, the heating time is longer than 3 hours,the heating temperature is dropped, thereby making it possible tocontrol the crystal state of TiO₂(B). This ensures that the heatingcondition to obtain TiO₂(B) having d₀₀₁ of 6.22 Å or more and a peakintensity ratio I(200)/I(001) of 0.5 or less can be optionally searched.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A titanium oxide compound having a crystalstructure of a monoclinic titanium dioxide and satisfying the followingequation (I):d₀₀₁≧6.223 Å  (I) wherein d₀₀₁ is the spacing of the (001) plane whichis measured by the powder X-ray diffraction method using a Cu—Kαradiation source, and wherein the titanium oxide compound has a crystalstructure which belongs to the space group C2/m, the titanium oxidecompound is suitable as an active material in a negative electrode of anonaqueous electrolyte battery, the titanium oxide compound has aparticle form the titanium oxide compound has a BET specific surfacearea in the range of 6 to 200 m²/g; and the titanium compound satisfiesthe following equation (II):0.39≦I(200)/I(001)≦0.5   (II) wherein I(001) is the peak intensity ofthe (001) plane and I(200) is the peak intensity of the (200) plane,which are measured by the powder X-ray diffraction method using a Cu—Kαradiation source.
 2. A nonaqueous electrolyte battery comprising: apositive electrode; a negative electrode comprising the titanium oxidecompound according to claim 1 as negative electrode active material; anda nonaqueous electrolyte.
 3. A battery pack comprising the nonaqueouselectrolyte battery according to claim
 2. 4. The battery pack accordingto claim 3, comprising a plurality of nonaqueous electrolyte batteries,wherein the nonaqueous electrolyte batteries are electrically connectedin series and/or in parallel.
 5. The nonaqueous battery according toclaim 2, wherein the positive electrode comprises a positive electrodeactive material and the positive electrode active material comprises atleast one selected from the group consisting of manganese dioxide, ironoxide, copper oxide, and nickel oxide impregnated with lithium,lithium-manganese composite oxide, lithium-nickel composite oxide,lithium-cobalt composite oxide, lithium-nickel-cobalt composite oxide,lithium-manganese-cobalt composite oxide, lithium-manganese-nickelcomposite oxide having a spinel structure, lithium-phosphorous oxidehaving an olivine structure, iron sulfate, vanadium oxide andlithium-nickel-cobalt-manganese composite oxide.
 6. The nonaqueousbattery according to claim 5, wherein the positive electrode activematerial has a specific surface area in the range of 0.1 to 10 m²/g. 7.The nonaqueous battery according to claim 2, further comprising apositive electrode current collector made of an aluminum foil or analuminum alloy foil.
 8. The nonaqueous battery according to claim 2,wherein the negative electrode further comprises at least one selectedfrom the group consisting of: titanium dioxide having an anatasestructure, lithium titanate having a rhamsdelite structure and lithiumtitanate having a spinel structure.
 9. The nonaqueous battery accordingto claim 2, further comprising a negative electrode current collectormade of copper, nickel, stainless or aluminum.
 10. The active materialaccording to claim 1, wherein d₀₀₁ is 6.50 Åor less.
 11. The titaniumoxide compound according to claim 1, wherein the crystal structure ofthe titanium oxide compound has a tunnel structure.
 12. The titaniumoxide compound according to claim 1, wherein d001 is 6.239 Åor less. 13.The titanium oxide compound according to claim 1, wherein I(200)/I(001)is in the range of 0.39 to 0.43.